Exceptions Aren't Always Errors

File CountLetters.java contains a program that reads a word from the user and prints the number of occurrences of each letter in the word. In reviewing the code, note that the word is converted to all upper case first, then each letter is translated to a number in the range 0..25 (by subtracting 'A' or decimal 65) for use as an index. No test is done to ensure that the characters are in fact letters.

1. Run CountLetters and enter a phrase, that is, more than one word with spaces or other punctuation in between. It should throw an ArrayIndexOutOfBoundsException, because a non-letter will generate an index that is not between 0 and 25. It might be desirable to allow non-letter characters, but not count them. Of course, you could explicitly test the value of the character to see if it is between 'A' and 'Z'. However, an alternative is to go ahead and use the translated character as an index, and catch an ArrayIndexOutOfBoundsException if it occurs. Since you want don't want to do anything when a non-letter occurs, the handler will be empty. Modify this method to do this as follows:

Put a try/catch inside the first for loop.

Add a catch that catches the eception, but don't do anything with it.

1. Now modify the body of the catch so that it displays a useful message followed by the character that is not a letter:

Not a letter: $

// ****************************************************************************

// CountLetters.java

// ****************************************************************************

import java.util.Scanner;

public class CountLetters

{

   public static void main(String[] args)

   {

      int[] counts = new int[26];

      Scanner scan = new Scanner(System.in);

      //get word from user

      System.out.print("n   Enter a single word (letters only): ");

      String word = scan.nextLine();

      //convert to all upper case

      word = word.toUpperCase();

      //count frequency of each letter in string

      for (int ; i < word.length(); i++)

         counts[word.charAt(i)-'A']++;

      //print frequencies

      System.out.println();

      for (int ; i < counts.length; i++)

         if (counts [i] != 0)

         System.out.println((char)(i +'A') + ": " + counts[i]);

   }

}